In roleplaying, you could easily imagine a char doing all that stuff at the same time.
I mean you can drop the sword/shield that you're holding in a split second, while at the same time removing your boots with just your feet and wiggling your head to make the metal cap fall off on its own ;)
Also, I think the way it is right now allows for a more streamlined gameplay.
Your idea, even if it makes perfect sense, would introduce additional calculations that a player has to make on top of a dozen other calculations, like PC's speed vs monster speed, range of projectiles and diagonal/straight paths for bolt spells etc.
I think this would only serve as an added complexity that doesn't really make the game more attractive.
Besides how would you address the impact of various attribute modifiers that technically should influence the speed of putting items on or taking them off?
Thin and nimble corruption would probably make it easier, stiff muscles - worse, PC's speed and dexterity would surely have their impact too.
I think overall that's a lot of extra coding for little actual content gain.

It probably would not be that much extra coding. On another note, I kind of like your idea of dexterity affecting the speed of changing equipment. It could be a [dexterity]% decrease in the energy cost or something.

Not sure about this one. On one hand, it doesn't make sense that changing the body armor takes many turns and changing the others takes zero, and that changing your ammunition also takes turns... On the other hand, neither does it make sense that you change your body armor in the same time as you eat a titan. (Neither does it make sense that you actually are able to eat a titan.) Generally I like consistency and it's really arbitrary to me which equipment changes are timeless and which are not.

Well, it's not really in the blink of an eye. If memory serves one turn at 1000 energy cost and 100 speed is around 31 seconds.
Which is acting once per 10 segments.

Still, one can imagine that it should take a little longer to change 13 pieces of equipment. Maybe increasing the energy cost could be another way to go.

Energy cost:= (weight of items removed and equipped)*constant.

Does ADOM need this? Well body armour weight is already used to determine equipping time.

Edit - Appologies Singbird if my post seems similar in some ways to yours, I promise I was typing while you posted. I was thinking before I typed though.

Edit 2 - Changing items other than missiles and body armour takes a turn, not?

Erm, yeah, it does. But just for the sake of not appearing dumb I will say that I meant that if you change multiple pieces, the ones after the first one are free. : )

I'm not too fond of weight-based times though, because it penalizes the early game. It also probably makes sense that changing a helm and changing gauntlets takes about the same time. Taking off and putting on gauntlets is quite slow after all. Similarly, putting on a girdle probably is quite slow, probably quite comparable to putting on a body armor. You cannot just slap it on make it go the right way like you can with a helmet or boots.

But I also think this could be a nice change tactically.

I like the current way this works. Too much realism can hurt the game.

I second anon123